1. Field of the Invention
This invention relates to a method for removing to relatively low concentration an acid component, such as H.sub.2 S, from a gaseous mixture. More particularly, the invention is concerned with the selective removal of H.sub.2 S from a stream which contains both H.sub.2 S and CO.sub.2.
2. Description of the Prior Art
In petroleum refining operations, the feedstock may contain sulfur compounds. Prior art methods for removing an acidic component, such as H.sub.2 S, are well-known and frequently have involved passing the acid-containing gaseous mixture upwardly through an absorber while a solution capable of absorbing significant quantities of the acidic components is passed downwardly. The acid-gas containing solution then is desorbed by passing this solution into a regenerator operated at a lower pressure, where substantial quantities of the acidic components are flashed off and stripped with reboiler steam which flows up the regenerator countercurrent to the solution. Certain flow modifications to the regenerator can be made in which a major portion of the solution passed to the regenerator is subsequently removed from an intermediate point on the regenerator while only partially desorbed and is returned to an intermediate point on the absorber. The downwardly flowing absorbing solution passes to the regenerator base and is transferred to a reboiler, where additional quantities of the acidic components are stripped off. This severely stripped or lean solution is recirculated to the top of the absorber where, because of its low absorbed acid concentration, it is capable of removing acidic compounds from the countercurrently flowing gas to relatively low levels. It has been known that amine containing solutions have relatively high acid-absorbing capacities. U.S. Pat. No. 2,818,323 discloses that diethanolamine, dipropylamine, dipropanolamine, butylamine and dibutylamine absorb acid compounds. U.S. Pat. Nos. 4,094,958; 4,112,050; 4,112,051; and 4,112,052 indicate that sterically hindered amines have exceptional acid absorption capacities. U.S. Pat. No. 3,622,267 discloses the use of methyldiethanolamine (MDEA) in the absorbing solution. It has been found that certain amines, such a methyldiethanolamine, selectively absorb H.sub.2 S from a gaseous mixture in preference to CO.sub.2. The use of selective amines makes it economically attractive in certain instances to recover the sulfur from the H.sub.2 S. If the acid gas flashed off from the regenerator does not contain excessive amounts of CO.sub.2, the acid gas may be directed to a sulfur recovery plant where the H.sub.2 S is converted to elemental sulfur.
Utilizing prior art scrubbing systems, when the sulfur concentration in the treated gas exceeds certain predetermined limits, the amount of lean absorbing solution circulated to the top of the absorber can be increased by passing increased quantities of the absorbing solution through the reboiler or for a given lean solution flow rate, the stripping steam rate can be increased. These methods have certain disadvantages. The increase in lean solution and/or steam rate flow increases the reboiler duty, and hence the operating cost. The higher circulation rate of the lean solution results in additional quantities of CO.sub.2 being absorbed for certain amines, such as MDEA, resulting in the acid gas stream to the sulfur recovery plant having an increased amount of CO.sub.2.
Other gas scrubbing operations have involved dry systems. U.S. Pat. No. 2,818,328 describes a system for removing H.sub.2 S from a gaseous mixture by contacting the gas with an adsorptive solid which has been impregnated with an amine. Regeneration is accomplished by heating the particles to volatilize the H.sub.2 S. This patent does not suggest using a liquid solvent, such as lean absorbing solution, for regeneration, nor does the patent suggest using a conventional liquid scrubbing system upstream of the bed of adsorptive solid. As a result, the beds of adsorptive solid must necessarily be large and the regeneration costs high.
U.S. Pat. No. 3,491,031 is directed at the use of vapor phase impregnation of monoethanolamine (MEA) on activated carbon for the non-selective removal of both CO.sub.2 and H.sub.2 S from a gaseous mixture. The carbon bed is regenerated by passing MEA vapor through the bed to remove the adsorbed compounds. The excess MEA then preferably is removed by passing an inert gas through the bed. There is no suggestion that the activated carbon bed should be used in conjunction with a conventional liquid scrubbing system, or that the activated carbon could be regenerated by passing a liquid solvent, such as a lean scrubbing solution through the bed. Here, also the activated carbon beds must necessarily be large and the regeneration costs high.